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Introduction to the Atomic
Model
Ms. Grobsky
Exploring the Black Box
Imagine you’ve
received a present that
you are not allowed to
open. You REALLY
want to know what’s
inside!
What might you
do to figure out
the contents?
Exploring the Black Box
In your group, try to figure out the contents and
internal structure your present WITHOUT opening it
Note: Do NOT shake the box too violently as the
contents may fall out!
As a group, be sure to answer the questions on the
hand-out!
So What is a Black Box?
A black box refers to any system that has
inner workings that cannot be directly
observed because it is not possible or
feasible
Can you think of any other black box
examples?
The Black Box and Science
An atom is an example of a black box
The Atomic Model and the Nature of
Science
When scientists wanted to determine the structure of
an atom, they were not able to actually see inside of the
atom
They had to rely on empirical evidence, indirect
evidence, observation, and inference when
developing their ideas-just as you did when
determining the contents of your present!
The Atomic Model and the Nature of
Science
Scientists developed a model to express their new
ideas
The understanding of the structure of the atom
has changed many times over the course of
centuries
As new evidence emerged, the ideas of the atomic
model changed
Lecturing about the scientists of the atomic model
isn’t quite as fun and interesting as understanding:
HOW was the atomic model developed?
WHY has the atomic model changed over the years?
The Evolution of the Atomic
Model
Democritus to Rutherford
Democritus
This is the Greek
philosopher Democritus
who began the search for a
description of matter more
than 2400 years ago
He asked: Could matter
be divided into smaller
and smaller pieces
forever, or was there a
limit to the number of
times a piece of matter
could be divided?
400 BC
Atomos
His theory: Matter could not
be divided into smaller and
smaller pieces forever,
eventually the smallest
possible piece would be
obtained
This piece would be
indivisible
He named the smallest piece
of matter “atomos,” meaning
“not to be cut”
Atomos

To Democritus, atoms
were small, hard particles
that were all made of the
same material but were
different shapes and sizes

Atoms were infinite in
number, always moving
and capable of joining
together
Democritus vs. The Philosophers
The eminent philosophers
of the time, Aristotle and
Plato, had a more
respected, (and ultimately
wrong) theory
Aristotle and Plato favored
the earth, fire, air and water
approach to the nature of
matter
Their ideas held sway
because of their eminence
as philosophers
The atomos idea was
buried for approximately
2000 years
Then Along Came Dalton’s Model
In the early 1800s, the
English chemist John
Dalton performed a
number of experiments that
eventually led to the
acceptance of the idea of
atoms
Dalton’s Atomic Model
All elements are composed of atoms
Atoms are invisible and indestructible
particles
Dalton’s Atomic Model
Continued
Atoms of the same element are
exactly alike
H
H
Dalton’s Atomic Model
Continued
Atoms of different elements are
different
O
H
Dalton’s Atomic Model
Continued
Compounds are formed by the joining
of atoms of two or more elements
H
H
O
Dalton’s Atomic Model
This theory became one of the
foundations of modern
chemistry
BUT, it needed work
Which of Dalton’s principles
do you know to not hold
true?
Thomson’s Plum Pudding Model
In 1897, the work of
an English scientist
J. J. Thomson
provided the first
hint that atoms
were made up of
even smaller
particles
Thomson Model
Thomson studied
the passage of an
electric current
through a gas
As the current
passed through the
gas, it gave off rays
of negatively
charged particles
Cathode Ray Tube
Animation
Thomson Model
This surprised
Thomson, because
the atoms of the
gas were
uncharged
Where did the
negative charges
come from?
• Thomson concluded that the
negative charges came from within
the atom
• A particle smaller than an atom had
to exist
• The atom was divisible!
• Thomson called the negatively
charged “corpuscles,” today known
as electrons
• Since the gas was known to be
neutral, having no charge, he
reasoned that there must be
positively charged particles in the
atom
• But, he could never find them!
And...
Thomson thought the positive (+)
and negative (-) particles were
s p r e a d o u t in atoms like:
ppuldudmisn
(plums in puddin’)
And Then There was
Rutherford…
In 1908, the English physicist Ernest Rutherford was
hard at work on an experiment that seemed to have
little to do with unraveling the mysteries of the atomic
structure